Added support for multiline helper declaration in source.tsv

You can now specify multiple lines of HTML in a helper declaration.  Multiline tags are wrapped like this: <<-START
  (one or more multiple lines of html content go here, including tabs if you want)
END->>

Email preprocesses the source.tsv to consolidate multiline declarations into a single line while retaining the formatting.  Added static constants to facilitate multiline decoding.  Updated read_properties to detect and process the start- and end-tokens.  Added a new email_spec to test multiline declarations.